Marine mammals
Marine mammals are popular among people. Unless a sperm whale strands, few animals receive so much publicity as the seal. And that is not only because of its sweet eyes. The seal is also an environmental indicator for the condition of its habitat. Not only seals live in the North Sea: porpoises and several species of dolphins also swim around here. Every once in awhile, a large whale will enter the North Sea. In the past, marine mammals were heavily hunted.
Back to the sea
Seals and whales are mammals. They are warm-blooded, they breath with the help of lungs and they deliver live births. The forefathers for all marine mammals were land animals. In the course of millions of years, they spent more and more time in the water, and adapted themselves to an aquatic life in all kinds of ways. Whales and dolphins are no longer able to live out of water. They need the upward force of the water to support their bodies, and even their young are born under water. Seals, on the contrary, go on land to give birth and nurse.
Just like fish, marine mammals have a streamlined body in order to swim well. The head is rounded and the protruding earlobes have disappeared. The front paws have been modified into paddle-like flippers. The seal's hind limbs have also been re-formed into flat flippers. By dolphins and whales, the hind legs are totally absent. The tail fin of whales is not an extension of the tail bone. It is a separately formed body part.
Because of their very thick layer of fat, marine mammals are able to keep warm in the cold water. This fat can also be used as an energy source during times of food scarcity. Seals have a remarkable manner for retaining warmth in places where the fat layers do not exist, for example, in the flippers: the arteries which bring the blood to the flippers are closely entangled with the arteries which return the blood from the flippers to the body. The blood flowing in releases a large part of its warmth into the blood returning from the flippers and thereby limits its heat loss. To prevent overheating in the summer, the flippers are then used for discharging warmth.
The seals have hair to prevent dehydration and damage to their skin when above water. Dolphins and whales have, and need, no hair since they always stay under water.
The eyes of marine mammals are poorly developed. Whales make use of sound waves to find their way about under water. Therefore, they are incredibly sensitive to noise interference below the sea surface. Seals have long, sensitive whiskers.
Scientists have discovered that marine mammals do not actively swim in order to dive to deep depths. Images from videa cameras attached to the back of various species of seals, whales and dolphins show that the animals only swim at the beginning of the dive. They trust gravity to do the rest and slowly glide down to the bottom. Because they use less energy in this way, they are able to stay longer under water.
At the top of the food pyramid
Marine mammals belong to the top of the marine food pyramid. At the base of this food pyramid are the microscopic plants, the phytoplankton; phytoplankton grow with the help of sunlight and nutrients. They are eaten by the planktonic animals, which float around in the seawater along with the phytoplankton. This zooplankton is eaten by benthic animals and small fish, which in turn are preyed upon by larger fish. And finally, these larger fish make up the menu for seabirds and marine mammals. The marine food chain ends with these animals.
The marine food chain is in reality more complicated than what has been described here. Baleen whales, for example, eat plankton as well as fish.
Fellow fishermen
Marine mammals are not the only 'fishermen' in the region of the North Sea. Professional fishermen catch almost three million tons of fish yearly in these waters, one quarter of the total stock in the North Sea! Sometimes, such very intensive fishing practices cause a food shortage for the marine mammals. There have been indications that the overfishing of herring in the 1960s affected the porpoise and dolphin population in the North Sea.
Many professional fishermen have never been very happy with the marine mammals. They consider the seals and porpoises competitors. Near Norway and Scotland, the amount of fish which the marine mammals consume is approximately 5% of the total fish yield. The percentage for the southern North Sea is less.
A seal eats 950-1200 kg of fatty fish per year. If it only eats flatfish, then it needs to eat 50% more. Therefore, the 4500 'Dutch' seals in 2002 consumed between 4200 to 5400 tons of fish (or 6400 to 8000 tons of flatfish). The entire seal population in the Wadden Sea in 2002 (around 21,000) ate 20,000 to 25,000 tons of fish per year (or around 35,000 tons of flatfish). This is still less than 1% of the three million tons caught by the fishermen.
Deadly nets
Marine mammals regularly swim into fishermen's nets, which usually ends in suffocation. Fykes catch many victims: in the Dutch coastal region, around 15 seals drown in fish fykes every year.
Especially the various types of drift nets catch many victims. These kilometer-long nets hang or stand as curtains in the water. During their search for fish, the marine mammals become entangled in the nets and are unable to get to the surface to breath. In 1987, harp seals migrated south from the Barentsz Sea in huge numbers, due to the overfishing in this sea. During their search for richer fishing grounds, 60,000 animals were entangled in standing nets and died. In the North Sea, 5000 to 8000 porpoises drown yearly in nets. Sometimes, dead or dying porpoises are found on the beaches with net marks embedded in their skin.
Fishermen sometimes lose sections of netting, for example, by accidentally motoring over one another's nets. These nylon nets rarely degrade and remain drifting for years in the sea. Marine mammals easily entangle in these ghost nets and drown. This marine litter is brought to land via the project 'Vuilvissen' (dirty fishing). A mound of 700 tons of discarded nets brought to the seal sanctuary in Pieterburen by fishermen was transformed into 'a complaint against the nonchalance whereby man treats nature'.
Research
For many years now, scientists have been researching the consequences of environmental pollution on marine mammals. They have determine that the reproduction of seals which have consumed PCB-polluted fish from the Wadden Sea is less successful than that of similar seals which are fed fish from the Atlantic Ocean. One has also shown that a relationship exists between environmental pollution and the seals' resistance to diseases. Such research is important in order to motivate the government for taking good environmental measures. Porpoises and dolphins in the North Sea have only been counted since the end of the 20th century. One estimates around 300,000 porpoises and more than 10,000 white-nosed dolphins in the North Sea. Beached animals are always examined for toxic materials.
Weblink
Marine mammal center:
http://www.tmmc.org/
World-wide organization of marine mammel centers:
http://www.ammpa.org/
Source: de Vleet, Ecomare
